[chumbucket]

From the sounds of what you described, you should be using two selector switches. Negatives are always common and do not pass through the selector switches. Only positive cable lead switching. Not clear on whether you want to be able to charge all four batteries or just two. It also sounds like you've got a two bank charger. They do make four bank chargers but they get a little pricey. I would think that for normal use during the boating season, you should only be concerned about charging the two trolling motor batts. During winter months, the batteries should be removed from the boat and stored indoors prefferably not on a concrete floor. Set them on some plywood or something. Then you can charge them on a rotating basis. I usually trickle charge them for one day per month. It's not good for batteries to drain to the point that the acid turns to water then freezing occurs.